1. Field of the Invention
The present invention is generally related to disk array architectures, and, specifically, to disk array architectures that provide disk fault tolerance.
2. Related Art
It is known to store data in an array of disks managed by an array controller to control the storage and retrieval of data from the array. One example of such a system is a Redundant Array of Independent Disks (RAID) comprising a collection of multiple disks organized into a disk array managed by a common array controller. The array controller presents the array to the user as one or more virtual disks. Disk arrays are the framework to which RAID functionality is added in functional levels to produce cost-effective, high-performance disk systems having varying degrees of reliability based on the type of RAID architecture implemented. RAID architecture can be conceptualized in two dimensions as individual disks arranged in adjacent columns. Typically, each disk is partitioned with several identically sized data partitions known as strips, or minor stripes. Distributed across the array of disks in rows, the identically sized partitioned strips form a data stripe across the entire array of disks. Therefore, the array contains stripes of data distributed as rows in the array, wherein each disk is partitioned into strips of identically partitioned data and only one strip of data is associated with each stripe in the array.
As is known, RAID architectures have been standardized into several categories. RAID level 0 is a performance-oriented striped data mapping technique incorporating uniformly sized blocks of storage assigned in a regular sequence to all of the disks in the array. RAID level 1, also called mirroring, provides simplicity and a high level of data availability, but at a relatively high cost due to the redundancy of the disks. RAID level 3 adds redundant information in the form of parity data to a parallel accessed striped array, permitting regeneration and rebuilding of lost data in the event of a single-disk failure. RAID level 4 uses parity concentrated on a single disk to allow error correction in the event of a single disk failure, but the member disks in a RAID 4 array are independently accessible. In a RAID 5 implementation, parity data is distributed across some or all of the member disks in the array. Thus, the RAID 5 architecture achieves performance by striping data blocks among N disks, and achieves fault-tolerance by using 1/N of its storage for parity blocks, calculated by taking the exclusive-or (XOR) of all data blocks in the parity disks row. A RAID 6 architecture is similar to RAID 5, but RAID 6 can overcome the failure of any two disks by using an additional parity block for each row (for a storage loss of 2/N). The first parity block (P) is calculated with XOR of the data blocks. The second parity block (Q) employs Reed-Solomon codes. One drawback of the known RAID 6 implementation is that it requires a complex and computationally time-consuming array controller to implement the Reed-Solomon codes necessary to recover from a two-disk failure. The complexity of Reed-Solomon codes may preclude the use of such codes in software, and may necessitate the use of expensive special purpose hardware. Thus, implementation of Reed-Solomon codes in a disk array increases the cost, complexity, and processing time of the array.
In addition, other schemes have been proposed to implement two disk fault protection. One such scheme, described in U.S. Pat. No. 6,353,895, calculates parity sets for rows and columns of the array. However, this implementation is limited to a square array architecture having the same number of stripes as the number of disks. Another implementation uses the same number of parity disks as the number of disks in the array, making the implementation cost prohibitive. Other schemes use computationally complex mathematical methods, such as Galois field multiplication and commutative ring computation methods. Still other schemes restrict the total number of disks in the array to be a number one less than a prime number.
Thus, it would be desirable to provide system and method for implementing a two disk fault recovery architecture that is not subject to the foregoing drawbacks. That is, it would be desirable to provide a system and method that are not subject to the complex and computationally time-consuming array control functions encountered in known two disk fault tolerance implementations. In addition, it would also be desirable to provide a method that does not limit the size or configuration of the array. Further, it would be desirable to limit the number of additional disks required to implement two disk fault tolerance.